<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 200px;
            height: 200px;
            background-color: pink;
            padding: 20px;
        }

        /* 实际大小：（200+20*2）^2=240*240 */
    </style>
</head>

<body>
    <div>
        盒子padding内边距影响盒子的实际大小。
    </div>
</body>

</html>